By virtue of its high level of crystallinity, trans-1,4-polybutadiene (TPBD) is a thermoplastic resin. Because it contains many double bonds in its polymeric backbone, TPBD can be blended and cocured with rubber. TPBD is similar to syndiotactic-1,2-polybutadiene in this respect. Even though the trans-1,4-polybutadiene of this invention is a thermoplastic resin, it becomes elastomeric when cured alone or when cocured with one or more rubbers.
Good molecular weight control can normally be achieved by utilizing an anionic polymerization system to produce TPBD. There is typically an inverse relationship between the catalyst level utilized and the molecular weight attained when anionic polymerization systems are used. Such an anionic polymerization system is disclosed in U.S. Pat. No. 4,225,690. The catalyst system disclosed therein is based on a dialkylmagnesium compound which is activated with a potassium alkoxide. However, such catalyst systems have not proven to be commercially successful.
TPBD is normally prepared utilizing transition metal catalysts or rare earth catalysts. The synthesis of TPBD with transition metal catalysts is described by J. Boor Jr., "Ziegler-Natta Catalysts and Polymerizations" Academic Press, New York, 1979 Chapters 5-6. The synthesis of TPBD with rare earth catalysts is described by D. K. Jenkins, Polymer, 26, 147 (1985). However, molecular weight control is difficult to achieve with such transition metal or rare earth catalysts and monomer conversions are often very modest.
Japanese Patent Application No. 67187-1967 discloses a catalyst system and technique for synthesizing TPBD consisting of 75 to 80% trans-1,4-structure and 20 to 25% 1,2-structure. The catalyst system described by this reference consists of a cobalt compound having a cobalt organic acid salt or organic ligand, an organoaluminum compound and phenol or naphthol. Gel formation is a serious problem which is frequently encountered when this three component catalyst system is utilized in the synthesis of TPBD. Gelation is a particularly serious problem in continuous polymerizations. By utilizing the catalyst system and techniques of this invention, TPBD can be synthesized in a continuous process with only minimal amounts of gel formation.
U.S. Pat. No. 5,089,574 is based upon the unexpected finding that carbon disulfide will act as a gel inhibitor in conjunction with three component catalyst systems which contain an organocobalt compound, an organoaluminum compound and a para-alkyl substituted phenol. U.S. Pat. No. 5,089,574 also indicates that conversions can be substantially improved by utilizing para-alkyl substituted phenols which contain from about 12 to about 26 carbon atoms and which preferably contain from about 6 to about 20 carbon atoms.
U.S. Pat. No. 5,089,574 more specifically reveals a process for synthesizing trans-1,4-polybutadiene in a continuous process which comprises continuously charging 1,3-butadiene monomer, an organocobalt compound, an organoaluminum compound, a para-substituted phenol, carbon disulfide, and an organic solvent into a reaction zone; allowing the 1,3-butadiene monomer to polymerize in said reaction zone to form the trans-l,4-polybutadiene; and continuously withdrawing the trans-1,4-polybutadiene from said reaction zone.
The techniques described in U.S. Pat. No. 5,089,574 are very useful in improving conversions and reducing gel formation. However, its teachings do not describe a technique for controlling the molecular weight of the TPBD being synthesized. In many applications, it would be desirable for the TPBD produced to have a lower molecular weight. There is, accordingly, a need to control the molecular weight of the TPBD produced with such ZiegleroNatta catalyst systems.